when driving in overdrive and under a load (maintaining speed or accelerating up a hill) the car bogs down and runs extremely rough, most of the time i can pull it out of it by downshifting and getting rpm's back. the check eng light flashes for the duration of the poor running. codes thrown are for cyl 5 misfire, the downstream o2 sensor and evap failure. plugs and wires are new but havent changed the runability. i get no response by tapping the TPS (an easy check i read about). could it be a cracked intake? it has the 3800 series II i believe.

there are a lot of things to look at for your driveability issue. Evap usually won't affect driveability unless it's really bad. (Like vacuum won't turn off and you collapse your gas tank... just kidding, that's very rare.)

The check engine light flashing means that your condition could damage your cat. So most likely the misfire on #5. since the problem is when it's under a load, the things you should look at first is secondary ignition. since you have already done the plugs and wires, How does the engine do when you power brake it? does it miss at all? Next thing would need to check your coils. If you have three sets of coils with two electrodes each, you could switch tow of them around. Switch the one that does #5 with another one. Then drive again and see if the problem is still #5 or did it move with the coil... If the problem is still #5 then maybe you should try switching #5 injector with another cylinder. If the problem moves with the injector it's probably a bad injector. Those are things you can try yourself before taking it to a shop. Easily verifiable items.

Now, if it isn't secondary ignition or a fuel injector, then you're going to need some equipment to check the signals for your coil, injector pulse. Then it could be mechanical too. You should look at the freeze frame to see what the computer was doing when the check engine light came on. Fuel trims and O2 sensors. you may want to drive with the scanner on to graph the data stream to see what is happening when it's missing.

what was the actual code number for the O2 sensor? then you have to check if the O2 sensor caused the problem of if the O2 sensor is responding to the problem.